The present application relates to a drive arrangement for a mobile, in particular hand-guided or portable, working machine for agriculture, forestry and horticulture applications, comprising an electric motor, a storage means for electrical energy (e.g., a DC power source such as a battery), and a control apparatus for the electric motor which is fed from the DC power source, with the electric motor having a specific, in particular linear, characteristic curve between the electric current which is consumed by the electric motor and the rotational speed of an output shaft of the electric motor, with the control apparatus having a current-limiter which limits the current which flows through the electric motor to a maximum value in order to protect the electric motor against overloading.
This application also relates to a corresponding method for controlling an electric motor, and also to a hand-guided working machine for agriculture, forestry and horticulture applications comprising a drive arrangement of this type.
In virtually all fields of electrical engineering, it is known to use current-limiting means for the purpose of protecting current-carrying components against overloading. These current-limiting means first take effect when the current exceeds specific, predefined limits (for example due to failure of other components, due to incorrect operator control operations etc.). In other words, the current is therefore generally limited to a maximum value independently of the applied voltage and/or independently of the respective electrical resistance.
Document DE 34 22 485 A1 discloses using a temperature-compensated current-limiting means in an electric motor which is intended to drive hand tools and the like. This document proposes measuring the electric current which flows through the motor by means of a current series shunt. Secondly, the temperature of the series shunt is measured by means of an NTC resistor. The temperature of the electric motor is indirectly detected from the temperature of the series shunt. When a critical motor temperature is reached, the current supply is reduced continually, but in a way which can be sensed by the user. In response to this, the user removes the load from the tool, so that the electric motor can rapidly cool down again.
In the field of working machines for agriculture, forestry and horticulture applications, machines which are driven by an internal combustion engine are known. These machines are frequently used in the professional sector. Furthermore, working machines which are driven electrically are known. In this case, it is known, particularly in the field of private use, to feed the electric motor from the mains. This requires connection of a stationary mains plug to the working machine via a cable. Machines of this type generally cannot be used in the professional sector since a mains plug is not available within an appropriate distance. Working machines which are fed from an electrical energy storage means (for example, a NiCd rechargeable battery) are known in the case of electrically driven working machines, both in the private field and in the professional sector. However, rechargeable lithium-based batteries have been used for several years. Rechargeable batteries of this type are generally also suitable for professional working machines, see WO2005/053915 (corresponding to US 2007/010 8944 A1). This document proposes using a current limiter to protect a lithium-based battery.